An atomic force microscope (AFM) scans over the surface of a sample. Typically, in the "contacting mode" of operation, a sharp tip is mounted on the end of a cantilever and the tip rides on the surface of a sample with an extremely light tracking force, on the order of 10.sup.-5 to 10.sup.-10 N. Profiles of the surface topography are obtained with extremely high resolution. Images showing the position of individual atoms are routinely obtained. In a second mode of operation, the tip is held a short distance, on the order of 5 to 500 Angstroms, from the surface of a sample and is deflected by various forces between the sample and the tip; such forces include electrostatic, magnetic, and van der Waals forces.
Atomic force microscopy is capable of imaging conductive as well as insulating surfaces with atomic resolution. Typical AFM's have a sensitivity of 0.1 Angstrom in the measurement of displacement, and a spring constant of about 1 Newton per meter (1N/m). Further, the cantilever must be mounted so that the cantilever can approach and contact a sample.
Several methods of detecting the deflection of the cantilever are available which have sub-angstrom sensitivity, including vacuum tunneling, optical interferometry, optical beam deflection, capacitive and resistive techniques. One such technique is described in PCT Patent Document WO 9212398, "Piezoresistive Cantilever For Atomic Force Microscopy", published Jul. 23, 1992 (PCT Application No. 91US9759), which is incorporated herein by reference.
There has been prior work in the field of near-field scanning optical microscopy. N. Van Hulst, M. Moer, O. Noordman, R. Tack, F. Segerink and B. Bolger have demonstrated a system for a near field scanning optical microscope using a microfabricated silicon-nitride probe integrated on a cantilever as originally developed for atomic force microscopy. N. Van Hulst, M. Moer, 0. Noordman, R. Tack, F. Segerink and B. Bolger, "Field Optical Microscope Using a Silicon Nitride Probe," Applied Physics Letter, Vol. 62, No. 5, 1 Feb. 1993, pp. 461-463. The fixed cantilever design disclosed allowed for routine close contact near field imaging on arbitrary surfaces without tip destruction. This alternative near field microscope utilized a photomultiplier and pinhole adjustment means in the imaging plane to collect light propagated at the probe apex. Light scattered in response to the probe disturbing an induced field of an evanescent wave generated on a glass substrate beneath the sample was focused and then collected at a distance from the sample. Accordingly, light generated by frustrated total internal reflection (FTR) at the apex to be collected by a photomultiplier was utilized to reveal sub-Angstrom topography for a given sample.
The previously cited prior art methods for collecting light generated by FTR are cumbersome and inefficient. Experimentation by the inventors of the present invention revealed that the prior art near-field optical microscope with silicon nitride cantilever probe of Hulst et al. described above, transmits a majority of the light to the photomultiplier through the nitride cantilever. Experimentation by the inventors of the present invention also revealed that by placing a metallic gold coating over the probe no signal appeared at the photomultiplier, even for relatively tall probe tips. If light were being scattered around the cantilever by the probe tip, then the gold coating would have had little effect on the photomultiplier signal. Similarly, it was discovered that the signal level at the photomultiplier is affected more strongly by tip radius than by exponential decay of the evanescent field. Accordingly, most of the light being transmitted to the photomultiplier for detection is in fact transmitted through the nitride cantilever. Understanding this relationship allows for an improved detection and collection scheme over that described in the prior art.
The present invention improves on the prior art near field scanning optical microscopes and atomic force microscopes by providing a conventional atomic force microscope including a conventional cantilever having a probe and displacement means for performing either "non-contact" or "contact" mode measurements. In addition the present invention includes an integrated photosensitive element embedded in the cantilever for efficiently detecting light generated at the probe apex and transforming said light energy into electrical signals for ease of amplification and transmission in signal analysis. The present invention eliminates the need for lenses, pinholes and photomultipliers. The present invention has demonstrated resolution well beyond the limits of diffraction, with resolution as high as one seventieth (1/70th) of an optical wavelength.